Regarding its principal features, the stock feed at a paper machine is generally as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers, the stocks are fed into stock chests, and from the stock chests further into a common blend chest, in which the stock components are mixed with each other. From the blend chest, the stock is fed into a machine chest, and from the machine chest there is an overflow back into the blend chest.
From the machine chest, the stock is fed into a dilution part of the wire pit, in which the stock is diluted with white water recovered from the wire section and serving as dilution water. From the wire pit, the stock is fed through one or more centrifugal cleaners into a deaeration tank. From the deaeration tank, stock free from air is fed through a machine screen into the headbox, i.e., into the inlet header thereof, and through the slice opening of the headbox to the wire section. A bypass flow of the headbox is fed back into the deaeration tank, and the white water recovered from the wire section is fed into the wire pit.
The basis weight and the ash content of the paper are measured on-line right before reeling from a ready, dry paper, usually by means of measurement apparatuses based on beta radiation and x-radiation. Based on this measurement, the basis weight of the paper is regulated, for example, by means of a so-called basis weight valve by whose means the stock flow after the machine chest is controlled. A second possibility is regulation of the speed of rotation of the pump that feeds stock from the machine chest into the wire pit. The ash content is controlled by dosing of fillers. The basis weight profile of the paper in the cross direction is obtained when the measurement apparatus is installed to move back and forth across the web.